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Geometry of variational methods: dynamics of closed quantum systems
by Lucas Hackl, Tommaso Guaita, Tao Shi, Jutho Haegeman, Eugene Demler, J. Ignacio Cirac
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Submission summary
Authors (as registered SciPost users):  Lucas Hackl · Jutho Haegeman 
Submission information  

Preprint Link:  https://arxiv.org/abs/2004.01015v2 (pdf) 
Date submitted:  20200714 02:00 
Submitted by:  Hackl, Lucas 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We present a systematic geometric framework to study closed quantum systems based on suitably chosen variational families. For the purpose of (A) real time evolution, (B) excitation spectra, (C) spectral functions and (D) imaginary time evolution, we show how the geometric approach highlights the necessity to distinguish between two classes of manifolds: K\"ahler and nonK\"ahler. Traditional variational methods typically require the variational family to be a K\"ahler manifold, where multiplication by the imaginary unit preserves the tangent spaces. This covers the vast majority of cases studied in the literature. However, recently proposed classes of generalized Gaussian states make it necessary to also include the nonK\"ahler case, which has already been encountered occasionally. We illustrate our approach in detail with a range of concrete examples where the geometric structures of the considered manifolds are particularly relevant. These go from Gaussian states and group theoretic coherent states to generalized Gaussian states.
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Author comments upon resubmission
List of changes
Detailed list of changes (including a pdf with changes marked in red) can be found in our reply to the referee.
Submission & Refereeing History
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Reports on this Submission
Anonymous Report 1 on 202098 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2004.01015v2, delivered 20200908, doi: 10.21468/SciPost.Report.1972
Strengths
1A thorough exposition of the geometry of variational methods in quantum mechanics with a focus on the difference between Kähler and nonKähler manifolds of variational wave functions.
2Simple examples help illustrate basic concepts.
Weaknesses
1The paper is long, maybe unnecessarily long, and not always concise.
2It is not always clear what the original contribution of this work are.
Report
I broadly agree with the first report on this paper. The manuscripts provides a link to, and an exposition of, mathematical concepts that may be useful for various applications of variational methods in studies of closed quantum systems. The paper attempts to give a detailed and comprehensive discussion of these methods, in a way accessible to a relatively broad expertise. For these reason I believe the paper could have impact and should be considered for publication in SciPost Physics.
As a consequence of the aim to be comprehensive, maybe, the paper is quite long. I think that in principle the length is not a problem, but for such a long paper to be more likely to have an impact, one would have wanted the writing to be a bit more concise and focussed. For example, there are several sentences and paragraphs that discuss what will be done in the next section or in some other part of the paper, without actually adding anything to the story at that point. This has the effect on the reader (at least this reader) that they loose focus and it sometimes becomes hard to come back to the paper and find the information one is after.
I do not think I will insist on the paper being shortened, but I would encourage the authors to consider attempting to focus the writing a little bit. Since how that would be done is to a large extend matter of style, I don't think I should give specific instructions. What would be useful for a reader that maybe wants to use the result of the paper rather than reading every word, is to have a more detailed guide to the reader for where and how to find the key results. I also find that the summary and discussion could be used more efficiently to help the reader understand the key points of the paper. At the moment the summary reads more like the introduction (especially due to the chose of tense in some parts) and doesn't actually summarise very much. It may be useful to extend this section to ease the use of the paper. Again, I would probably not insist on these changes, as they are to some extend a matter of style, but I feel it would improve the readability of the paper.
Towards the end of the paper there is a discussion of the Generalized Gaussian states, and it is said that one of the main motivation of this work is to understand the nontrivial manifolds that arise from these states. There is also several references to earlier work on these states and it is not always clear what is fully new, what is described in a new way and what was discussed before. It would be useful to clarify this.
In addition to this I give a list of more detailed points for consideration in the Requested changes section.
Requested changes
Some of these points are more remarks than requested changes.
1There are a few places, such as in the last sentence of paragraph 2 on page 2, which reads "Recently such methods have been used..." and then gives examples, where references are partially or completely missing. In this particular paragraph, for example, reference should be give to these recent works that are mentioned. Another example is start of IIB.3 "A standard approach..." Such a phrase seems to suggest that a reference would be appropriate.
2First paragraph of section II reads "Readers may skip directly to section III." I suppose that this is an example of a guid to the reader I mentioned in my main report, except that here it is not useful. What readers may skip directly and for what reason should they do that?
3I find the first four paragraphs of section III to be quite repetitive. Maybe one place to shorten?
4There are several places where there is an additional article or a missing article. For example in second paragraph of III.A: "...where $\psi(z)\rangle \in \mathcal{H}$ is a holomorphic in $z^\mu$..."
5The definition of $\omega_{\mu\nu} $ given just below Eq. (8) and that in footnote 2, do not seem consistent. They seem to differ by an $i$.
6Below Eq. (20) a $\bar{v}_\mu\rangle$ is used. I may have missed it, but I don't think the bar notation has been defined.
7In the paragraph below Example 2, should $\mathcal{P}(\mathcal{P})$ maybe be $\mathcal{P}(\mathcal{H})$?
8Above Eq. (26) where the authors state that some things can be shown to be equivalent, is this a known result (and therefore maybe needing a reference) or is it something they can show but don't think is needed to expand on in the paper?
9At the start of III should $\psi$ be $\psi\rangle$.
10Not all figures and tables seem to be referenced in the main text (for example Fig. 2). It would make sense to refer to the figures in the correct place. Also, I feel that sometimes the captions could be more detailed, as often not everything that is shown on the figures is defined or explained (for example caption to Fig. 3).
11Should definition 1 read "...with inverse $\Omega^{\mu\nu}$..."?
12Is there an superfluous dot at the end of Eq. (41)?
13Start of IV: "Given a system $\mathcal{H}$" Here the concept of a system is being mixed with Hilbert space, since $\mathcal{H}$ is everywhere else used for that.
14First sentence in the paragraph after Eq. (87): "In the case of the Lagrangian action principle...takes the following form." Nothing follows this, only the next sentence "A similar derivation..." What form does it take?
15Last paragraph on page 16. "full Hilbert space, i.e., $\mathcal{M} = \mathcal{P}(\mathcal{H})$." Wasn't $\mathcal{P}(\mathcal{H})$ the projected Hilbert space?
16Above proposition 5, again a sentence that ends in "take the following form" without any form following.
17Below Eq. (98), should it be $\check{J}$ that "clearly satisfies $\check{J}^2=1$?
18IVB.2 starts with "A common alternative is..." A common alternative to what?
19Above Eq. (19) the authors use $\mathcal{F}^i$ while elsewhere it is $\mathcal{F}^\mu$.
20At the top of page 32, should the "associated tangent vector" be $\delta\Gamma\rangle$ instead of $\Delta\Gamma\rangle$?
21In Eq. (265) one of these matrices should probably be $\delta\Gamma_2$.
22In example 20, should some of the $q$ and $p$ have subscript 2 instead of 1. And also, is there a reason the order in $\xi$ is $q,p,q,p$ instead of $q,q,p,p$ as earlier in the paper.
23In Eq. (292) would it make sense to show explicitly that the derivative is taken at $x=0$?
24Proposition 13 says "The restricted Kähler structures are..." The restricted Kähler structures of what? It would be preferable to make the propositions selfcontained.
25In proposition 14, is is needed to write that $\mathcal{G}$ is a Lie group?
26End of example 23. What balance between the properties of $\mathcal{M}_\phi$ and its dimension needs to be struck and for what purpose?
27Example 24. "The representation of the representation on $\mathcal{H}_$"?
28First sentence in Example 25 seems unnecessary since the whole section has been discussing this.
29Appendix A.3 starts with "In many areas of physics, ...". Then there is a heading and discussion goes somewhere else. The connection of this first statement to what follows is not clear.
30In couple of places there are additional articles "we a reference", "we a corresponding" ...
Author: Lucas Hackl on 20200921 [id 980]
(in reply to Report 1 on 20200908)We thank the referee for the careful reading of our manuscript and his/her suggestions for improvement which we were very happy to implement. Please find a revised version of the manuscript attached, where we marked the respective changes in red. Let us add some comments regarding some of these changes. We will also resubmit the revised version to arXiv (without markup) and SciPost.
Let us comment more specifically on the referee’s general comments:  “For example, there are several sentences and paragraphs that discuss what will be done in the next section or in some other part of the paper, without actually adding anything to the story at that point.” We believe that the referee particularly refers to the introductory paragraphs of section II, III and IV, which we rewrote to be more of a guide to the reader than a summary.  “What would be useful for a reader that maybe wants to use the result of the paper rather than reading every word, is to have a more detailed guide to the reader for where and how to find the key results.” “I also find that the summary and discussion could be used more efficiently to help the reader understand the key points of the paper. At the moment the summary reads more like the introduction (especially due to the chose of tense in some parts) and doesn't actually summarise very much.” We already partially addressed this by giving more instructions to the reader in the more compact summary paragraphs to the main sections, but we significantly rewrote the “Summary and Discussion” section to follow the suggestions of the referee. We now use past tense for the summary and highlight what we believe to be the main contributions of our paper. In particular, we try to be more explicit regarding our contributions. We believe that the main overall contribution is to highlight the Kähler property as an important criterion for variational families (which has been only implicitly used in the past by typically restricting to the Kähler case straightaway without often saying so) and the presentation of a systematic geometric framework.  “Towards the end of the paper there is a discussion of the Generalized Gaussian states, and it is said that one of the main motivations of this work is to understand the nontrivial manifolds that arise from these states. There is also several references to earlier work on these states and it is not always clear what is fully new, what is described in a new way and what was discussed before. It would be useful to clarify this.” This is an important point, which we addressed in section V.C. Generalized Gaussian states were introduced as an ansatz for manybody wave functions, but their group theoretic and geometric properties were not fully understood. The present paper defines them in grouptheoretic language and highlights the fact that they are nonKähler manifolds, which makes them a prime example for the application of the nonKähler methods presented in the paper.
We also addressed the numbered list of items and could resolve most of them directly. Let us briefly comment on the following items: 3. As discussed previously, we rewrote this paragraph to serve as a useful guide to the reader and significantly shortened it to avoid repetition. 8. This is shown in the paper and we now explicitly refer to the place where we do it. 9. The letter \psi without \ket{...} is correct. We use the letter \psi without \ket{...} if we refer to the element in projective Hilbert space, i.e., the equivalence class of states as defined in (41) of section III. While we make sure to stay consistent with this convention, we did not want to be overly pedantic by emphasizing this point too much. 22. We made the numbering of the variables (q1,...,qN,p1,...,pN) consistent throughout the draft. 23. G needs to be a compact Lie group, which we stated in the proposition, so we did not change this. 26. We removed this sentence from the example and moved it instead to the discussion section. There, we emphasize that a good variational family must strike a balance between being large enough to capture the relevant physics, but small enough to decrease computational complexity (compared to the exponentially large full Hilbert space) to be able to do calculations.
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